Optical wellbore fluid characteristic sensor

ABSTRACT

Evanescent waveguide sensors for measuring and determining the composition of wellbore fluids in situ are provided. The waveguides are provided on substrates have a thickness and strength sufficient to withstand wellbore pressures and a sufficient surface area to allow for broad range measuring of the wellbore characteristics. The optical sensors facilitate determination of the wellbore fluid composition without requiring in-tool sampling of the wellbore fluid.

FIELD OF THE INVENTION

The present invention relates in general to downhole wellbore loggingdevices and methods, and more particularly, to fiber optic sensors formeasuring wellbore fluid characteristics in situ.

BACKGROUND

In wellbore operations it is often desirable to identifycharacteristics, such as the composition, of the wellbore fluid atidentifiable locations in the wellbore. For example, in somecircumstances it may be desirable to identify at what location in thewellbore that water, gas, or oil is entering from the surroundingformation. It may further be a desire to identify the presence ofhydrogen sulfide or carbon dioxide.

It is a desire of the present invention to provide methods and apparatusfor determining the composition and/or characteristics of wellborefluids in the wellbore. It is a still further desire to provide opticalfiber sensors for determining wellbore fluid characteristics. It is astill further desire to provide optical sensors and methods fordetermining the chemical composition of a wellbore fluid, in thewellbore, without requiring in-tool fluid sampling.

SUMMARY OF THE INVENTION

In view of the foregoing and other considerations, the present inventionrelates to determining the composition of wellbore fluids using opticalsensors.

In an example of an optical apparatus for investigating wellbore fluidsof the present invention, the apparatus includes an optical sensorhaving a waveguide formed on the surface of a substrate; a portion ofthe waveguide is open to the surrounding environment to emit anevanescent sensing field therein. The substrate has a thicknesssufficient to withstand the pressures encountered in wellbores. Morethan one waveguide, or discreet sensor may be formed on the substrate.

An optical system includes one or more optical sensors carried by awellbore tool. The tool may be a logging tool or drilling tool. The toolmay carry one or more optical sensors. Multiple sensors may be formed ona single substrate. Desirably the optical sensor system of the presentinvention provides an economical, rugged optical sensor system that candetermine the composition of the wellbore fluid, as well as othercharacteristics, without in-tool sampling of the fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and aspects of the present inventionwill be best understood with reference to the following detaileddescription of a specific embodiment of the invention, when read inconjunction with the accompanying drawings, wherein:

FIG. 1 is a wellbore schematic wherein an optical sensor system of thepresent invention is deployed;

FIG. 2 is a side view of an example of a reflectance optical sensor ofthe present invention; and

FIG. 3 is a plan view of a refractive index optical sensor of thepresent invention.

DETAILED DESCRIPTION

Refer now to the drawings wherein depicted elements are not necessarilyshown to scale and wherein like or similar elements are designated bythe same reference numeral through the several views.

As used herein, the terms “up” and “down”; “upper” and “lower”; andother like terms indicating relative positions to a given point orelement are utilized to more clearly describe some elements of theembodiments of the invention. Commonly, these terms relate to areference point as the surface from which drilling operations areinitiated as being the top point and the total depth of the well beingthe lowest point.

Referring now to FIG. 1, an optical borehole composition sensor systemof the present invention, generally denoted by the numeral 10, isillustrated. A tool 12 is suspended in a well 14 or borehole by aconveyance 16. In the illustrated example, tool 12 is a productionlogging tool and conveyance 16 is a cable. Cable 16 may includeconductors (not shown), which may be electrical and/or optical, forcommunicating with data processing equipment 18. Tool 12 may be utilizedwith or incorporated into a logging or measurement while drilling tooland conveyed by tubing or drill pipe.

Tool 12 includes one or more optical sensors 20 for detecting andmeasuring various characteristics of the contents of the wellbore suchas, but not limited to, water cut of the wellbore fluid 26, gas holdup,oil composition, carbon dioxide content, hydrogen sulfide detection,temperature and pressure. An optical (light) source 23 and detectionequipment 25 are in operational connection with the one or more opticalsensors 20. The light source 23 and detection equipment 25 may belocated in various locations such as the surface 22 or in theelectronics housing 24 of tool 12.

Tool 12 includes one or more evanescent waveguide sensors 20 that areoperational in the harsh environment of wellbores. Evanescent sensors 20include reflectance, transmission sensors 20 a (FIG. 2) and refractiveindex sensors 20 b (FIG. 3). Note that sensors 20 are optical fibersensors and therefore are very small in relation to tool 12. As notedwith reference to FIG. 1, one or more sensors 20 may be carried by tool12. If tool 12 includes a plurality of sensors 20, the sensors may bepositioned proximate to one another or scattered along tool 12. Itshould further be recognized that sensors 20 may be positioned inwellbore 14 in various manners including the logging tool exampledescribed herein.

Sensors 20 include waveguides that are fabricated on the surface of asubstrate 28 (FIGS. 2, 3). The waveguides may be fabricated usingin-diffusion or ion implantation and both processes may be based onreproducible and standard photo-lithographical/planar processing. Thesemethods facilitate the accurate definition of both the waveguide pathand the size of the windows 32 (FIGS. 2, 3) to specify the properties ofthe sensor.

Various materials such as sapphire, silicon, silica and diamond may beutilized for the substrate. Sapphire provides benefits that aredesirable for utilization in wellbores. Some of the benefits of sapphirecrystals include low cost and availability in relatively large sizes.Size of the substrate is important for various reasons. For example, thethickness of substrate 28 must be sufficient to withstand the pressures,in particular the pressure differential across substrate 28, encounteredin wellbores. Sapphire substrates can readily be obtained in thicknessesof 6 mm, providing suitable strength for many wellbore applications.Although substrate 28 is illustrated and described in terms of planarconfigurations, cylindrical or hemispherical sapphire substrates 28 maybe utilize in particular for high pressure and/or high temperaturewells. Additionally, sapphire substrates 28 are available with largesurface areas (for example, diameters of 25 mm) providing for theplacement of a number of discreet sensors on each substrate.

Refer now to FIG. 2 wherein a side view of a reflectance sensor 20 a isillustrated. A sapphire substrate 28 is provided in connection with anoptic fiber 30. A window 32 is formed through the covering or overclad34 exposing a portion of the surface 36 of substrate 28 on which thewaveguide (40, FIG. 3) is formed. Window 32 exposes fluid 26 directly tothe evanescent field and forming a sensing field 38.

Sensor 20 a measures the spectral content of the waveguide transmission.Fluid 26 in contact with the evanescent field of the waveguide will havea characteristic absorption fingerprint corresponding to molecularabsorption bands that can be used to identify the constituent componentsof the fluid 26. The molecular absorption bands and/or the constituentcomponents of fluid 26 are communicated via display or the like to anoperator.

It is noted that the transmission should be interrogated over a relevantspectral band, for example by sweeping the wavelength of theilluminating light source and measuring the intensity on detector 25(FIG. 1). Tool 12 may include sensors 20 a having different lengths,from microns to millimeters, of windows 32 to cover a desired range oftransmission loss and sensitivity.

Refer now to FIG. 3, wherein an example of a refractive index sensor 20b is illustrated. Sensor 20 b is illustrated as a Mach-Zehnderinterferometer. Waveguide 40 includes a first branch 42 and a secondbranch 44 etched on the surface of substrate 28. Light travels alongwaveguide 40 in the direction indicated by the arrow. As indicated inFIG. 2, the majority of waveguide 40 is covered with overclad 36 so thatthe evanescent field does not penetrate into the wellbore fluid. Secondbranch 44 of waveguide 40 includes a window 32 in the overclad orcovering to expose waveguide 44 as illustrated in FIG. 2.

The interferometric configuration of sensor 20 b measures the differencein optical path length between two or more waveguides or waveguidebranches. Waveguide 42 is immune to the effect of fluid on its surface,as it is covered with an optical protective layer. Second waveguide 44has a window 32 of a specific chosen length so that the refractive indexof the fluid 26 in contact with waveguide 44 alters the optical pathlength which is detected with the interferometric arrangement. Themagnitude of the change in the optical path length can be used to assessthe nature and composition of wellbore fluid 26 in contact withwaveguide 44.

An example of operation is now described with reference to FIGS. 1through 3. Light is emitted from LED source 23 and travels along opticalfiber 30 protected from the downhole environment by cladding 34. Whenthe light arrives at window 32, some of the light interacts withwellbore fluid 26, and the remaining light is reflected and travels backthrough the optical fiber. The reflected light travels through a Ycoupler to a receiving photodiode 25 and is converted into an electricalsignal. The amount of reflection depends on the refractive index offluid 26.

From the foregoing detailed description of specific embodiments of theinvention, it should be apparent that a novel and unobvious system andmethod for downhole testing and determination of the composition andcharacteristics of a wellbore fluid has been disclosed. Althoughspecific embodiments of the invention have been disclosed herein in somedetail, this has been done solely for the purposes of describing variousfeatures and aspects of the invention, and is not intended to belimiting with respect to the scope of the invention. It is contemplatedthat various substitutions, alterations, and/or modifications, includingbut not limited to those implementation variations which may have beensuggested herein, may be made to the disclosed embodiments withoutdeparting from the spirit and scope of the invention as defined by theappended claims which follow.

1. An optical apparatus for investigating wellbore fluids, the apparatuscomprising an optical sensor having a waveguide formed on a surface of asubstrate, a portion of the waveguide being open to a surroundingenvironment to emit an evanescent sensing field therein.
 2. Theapparatus of claim 1, wherein the substrate is sapphire.
 3. Theapparatus of claim 1, wherein the optical waveguide further includes asecond waveguide portion that is coated with a protective layer suchthat the evanescent field does not penetrate into the surroundingenvironment.
 4. The apparatus of claim 3, wherein the substrate issapphire.
 5. An optical system for investigating a fluid in a wellbore,the system comprising a tool suspended in a wellbore and surrounded bythe fluid in the wellbore; and an optical sensor positioned on the tool,the sensor measuring at least one characteristic of the wellbore fluid,wherein the optical sensor comprises a waveguide formed on a substrate,the waveguide having a portion open to the surrounding wellbore fluid.6. (canceled)
 7. The system of claim 5, wherein the optical sensor is areflectance, transmission type sensor.
 8. (canceled)
 9. The system ofclaim 5, wherein the optical sensor is a transmissive type sensor. 10.The system of claim 5, wherein the substrate is diamond.
 11. The systemof claim 5, wherein the substrate is sapphire.
 12. The system of claim5, wherein the optical sensor is a reflectance type sensor.
 13. Thesystem of claim 5, wherein the optical sensor is a refractive index typesensor.
 14. The system of claim 7, further including a second opticalsensor carried by the tool, the second optical sensor being a refractiveindex type sensor.
 15. The system of claim 14, wherein the reflectancetype sensor and the refractive index type sensor are each formed on asingle substrate.
 16. The system of claim 15, wherein the substrate issapphire.
 17. A method of measuring the composition of a fluid in awellbore, the method comprising the steps of: providing a first opticalsensor on a tool, the optical sensor comprising a waveguide formed on asubstrate, an optically protective covering over the waveguide, and awindow formed through the protective covering exposing a portion of thewaveguide; disposing the tool into a wellbore and the wellbore fluid;and measuring at least one characteristic of the wellbore fluid.
 18. Themethod of claim 17, wherein the substrate is sapphire.
 19. The method ofclaim 17, wherein the first optical sensor is a reflectance type opticalsensor and the tool further including a second refractive index typeoptical sensor.
 20. The method of claim 19, wherein the second opticalsensor includes a waveguide formed on the same substrate as the firstoptical sensor.